Debt, Instrument, Interest Expense, Increase (Decrease)

No definition available.

Name:	cmls_DebtInstrumentInterestExpenseIncreaseDecrease
Namespace Prefix:	cmls_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The cash outflow for debt initially having maturity due after one year or beyond the normal operating cycle, if longer.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 15
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3291-108585

+ Details

Name:	us-gaap_RepaymentsOfLongTermDebt
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	us-gaap_CreditFacilityAxis=us-gaap_LineOfCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=cmls_CanceledCreditAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableRateAxis=us-gaap_LondonInterbankOfferedRateLIBORMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableRateAxis=us-gaap_BaseRateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableRateAxis=cmls_FederalFundsRateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Long-Term Debt (Canceled Credit Agreement) (Detail) - USD ($) $ in Thousands		5 Months Ended	7 Months Ended	12 Months Ended
	Dec. 23, 2013	Jun. 03, 2018	Dec. 31, 2018	Dec. 31, 2019
Debt Instrument [Line Items]
Repayments of long-term debt		$ 0	$ 56,500	$ 1,242,918
Term Loan \| Canceled Credit Agreement
Debt Instrument [Line Items]
Basis spread on variable rate	1.00%
Repayments of long-term debt		37,800
Interest expense (decrease) increase		$ (37,100)
Term Loan \| Canceled Credit Agreement \| London Interbank Offered Rate
Debt Instrument [Line Items]
Basis spread on variable rate	3.25%
Term Loan \| Canceled Credit Agreement \| London Interbank Offered Rate \| Minimum
Debt Instrument [Line Items]
Interest rate	1.00%
Term Loan \| Canceled Credit Agreement \| Base Rate
Debt Instrument [Line Items]
Basis spread on variable rate	2.25%
Term Loan \| Canceled Credit Agreement \| Base Rate \| Minimum
Debt Instrument [Line Items]
Basis spread on variable rate	2.00%
Term Loan \| Canceled Credit Agreement \| Federal Funds Rate
Debt Instrument [Line Items]
Basis spread on variable rate	0.50%